Automobiles and other combustion engine powered vehicles typically include an electric starting motor for starting the combustion engine for operation. In this regard, the starting motor is coupled to a starting circuit which generally receives electrical energy from an on-board electric storage battery. The starting circuit selectively couples electrical energy from the battery to the starting motor that operates to cycle the engine to initiate sustained operation. In common vehicle applications, the battery also provides electrical energy to a variety of electric power consuming devices such as engine control electronics, lighting systems, and vehicle accessories.
Traditional batteries for these applications, often referred to as starting, lighting and ignition (SLI) batteries, are multi-cell, lead-acid batteries. That is, the batteries are constructed from lead plates pasted with active material and arranged into stacks. The stacks are inserted into partitioned cell compartments of a battery container, electrically connected, and flooded with dilute acid electrolyte. SLI batteries of this construction are more than adequate for providing the relatively high power demand required of engine starting as well as the relatively low power demand to maintain electrical accessories during both vehicle operation and periods of non-operation. However, because of the seemingly disparate functions the SLI battery is required to perform, short duration high-power output and long duration low-power output, the battery design can not be optimized for performing either of these tasks. An additional drawback of these batteries is relatively low specific energy (kilo-watt hours/grams (kWh/g)) as compared to other battery constructions owing to the weight of the lead plates and the liquid electrolyte.
There has been suggested a battery system for vehicle use which includes two batteries. A first battery in the system, a starting battery, is optimized for engine starting, that is, designed specifically for short duration, high-power output. A second battery in the system, a reserve battery, is optimized for operating and maintaining non-starting electrical loads. An advantage of such a system is that the starting battery may be made smaller and lighter yet capable of provide a high power output for a short period of time. In addition, the reserve battery may be made smaller and lighter yet capable of satisfying the relatively low power requirements of the vehicle accessories. In combination, the two battery system may require less space and weigh less than a single traditional SLI battery.
A limitation of such a system lies with the starting battery. Small, light weight batteries capable of high operational discharge rates typically have high self-discharge rates. That is, left unattended, the starting battery will self-discharge to a level where it is incapable of providing sufficient electrical energy for starting the engine. During vehicle operation, the starting battery is charged using the vehicle electrical system. Therefore, where the vehicle is regularly used starting battery self-discharge is not a concern. However, if the vehicle is left unused for an extended period of time or the vehicle is used for very short trips during which time the vehicle electrical system does not sufficiently charge the starting battery, the starting battery may become discharged leaving the operator stranded.
Therefore, a dual-battery system for vehicle starting and operation that provides the advantages of reduced size and weight and yet overcomes the disadvantages of starting battery self-discharge is highly desirable.